1. Department of Solid-State Sciences, Ghent University, 9000 Gent, Belgium
2. Instituut voor Kern-en Stralingsfysica, KU Leuven, B-3001 Leuven, Belgium
3. EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
4. IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA